The present invention relates to an improved cryogenic cooler, and to a process chamber using such cooler.
More particularly, the invention provides a method of defrosting the cold head and the cooled panel of a Stirling-cycle cooler without requiring electric heaters for this purpose.
Cryogenic coolers are used in industrial, medical and research fields for various purposes such as gas liquefaction, sputtering processes, cooling superconducting magnets, cooling infra-red sensors, during the manufacture of semi-conductors, refrigerated storage of biological materials, X-ray detectors, and cooling radio-frequency antenna components.
The standard Stirling-cycle refrigerator consists of a piston for isothermal compression of the working fluid, usually helium, and a displacer, which can operate in the same cylinder with the piston, or it can operate in its own cylinder.
The displacer and piston are connected, either mechanically to the same driven shaft or merely by a gas conduit. When operating, the cylinder and displacer are usually displaced in phase by 90 degrees. The displacer pushes compressed gas isochorically from the warm region where it was compressed through the regenerator into the cold region where the compressed gas is expanded isothermally doing work on the displacer and producing irrgeration. The displacer returns the gas after expansion through the regenerator for re-compression by the piston.
The regenerator is cooled by gas having completed its expansion cycle and so is able to cool incoming gases from the next cycle before they enter the expander chamber. All Stirling machines use a regenerator to improve efficiency, although this item is not needed for gaining an understanding of the Stirling cycle.
Recent U.S. patents disclose suggested improvements to Stirling refrigerators.
In U.S. Pat. No. 5,502,968 Beale discloses a free piston Stirling machine having a variable power transmitting linkage connecting the displacer and piston. This adjustment is used in a cooler to control the thermal pumping rate.
Benschop in U.S. Pat. No. 5,590,534 proposes to add heat flow reduction means between the compressor and the cooling element which is claimed to eliminate the need for heat sinking on the warm side:
The NASA Ames Research Center reported a recent development in Stirling coolers concerning a Pulse Tube cooler at the 8th International Cryocooler Conference, 1994. This cooler is similar to the standard Stirling cooler, but there is no displacer. Instead, the working gas oscillates back and forth in the pulse tube, working at frequencies well below resonance. Heating results at the closed end of the tube and cooling at the end adjacent to the regenerator. Efficiency improvements have been made by adding a reservoir at the hot end of the pulse tube, and adding by-pass tubes between the regenerator and the pulse tube. The attractions of the pulse tube include few moving parts, and improved durability. The Pulse Tube cooler requires yet further efficiency improvement before becoming competitive with the standard Stirling or other known coolers such as the Gifford-McMahon type.
It is usually advantageous that cryogenic processes be carried out in a vacuum. One reason is to eliminate air convection and its resultant inward heat leakage.
Other reasons are connected with the particular process being carried out.
One of the problems encountered with cryogenic refrigerators is frosting over of the cold end, and eventually also of the cryopanel with which it is in thermal contact. During normal operation gas remnants freeze and solidify on the cryopanel, reduce its heat transfer coefficient to cause a deterioration in performance, and de-frosting needs to be carried outxe2x80x94a process familiar to owners of ordinary household refrigerators not provided with automatic defrosting. In certain processes this requires that the process be shut down, while electric heaters are operated to fast defrost both the cold end and the cryopanel. The use of electric heaters near cryogenic equipment can cause safety problems, and complicates the equipment layout making maintenance more difficult. Alternatively slow defrosting can be carried out simply by shutting down the cooler, but much valuable processing time is lost. Before resuming work on the process being carried out, the released gases need to be removed, for which purpose a turbomolecular pump is typically employed to reduce gas pressure to under 10xe2x88x929 torr, depending on the process requirements.
It is therefore one of the objects of the present invention to obviate the disadvantages of prior art Stirling coolers and to provide a device which can be fast defrosted without the need for electric heaters.
It is a further object of the present invention to provide a cryogenic vacuum process chamber where the cryopanel can also be fast defrosted without the need for electric heaters.
The present invention achieves the above objects by providing a Stirling cooler comprising a driven piston in a first cylinder for maintaining reciprocal gas displacement by compressing a gas in a closed cycle, said cylinder being in fluid communication through a conduit with a second cylinder containing a free displacer, said displacer normally oscillating inside said second cylinder in response to gas pulses received through said conduit. The second cylinder further contains a regenerator, and an expansion chamber being cooled by said gas at a first extremity.
A pneumatic spring volume on which work is performed is linked to the displacer at a second extremity. Heat rejection means are in thermal contact with the conduit.
Means for driving the piston at a speed required for normal cooling generation result in displacer oscillation being less than 90 degrees out of phase with the piston.
Means are provided for selectively driving the piston at a speed above the resonant frequency of the displacer to cause displacer oscillation to be out of phase with the piston by more than 90 degrees for the generation of heat in the expansion chamber.
In a preferred embodiment of the present invention the is provided a cooler wherein piston drive means comprise a variable speed electric motor.
In a most preferred embodiment of the present invention there is provided a cryogenic vacuum process chamber cooled by a panel in thermal contact with a cooler. The panel receives heat from said cooler for defrosting of said panel when said cooler is selectively operated at a speed above the resonant frequency.
Yet further embodiments of the invention will be described hereinafter.
In U.S. Pat. No. 5,813,235 Peterson describes and claims a resonantly coupled alpha Stirling cooler which has hot and cold variable-volume chambers, a regenerator, and a driver for maintaining reciprocating gas displacement between the chambers. Only the hot side of the cooler is driven. The cold side responds passively by resonant coupling. The phase difference between volume oscillations in the hot and cold variable-volume chambers is altered by adjusting the driving frequency.
From claim 10 it is clear that the aim of changing the drive frequency is to increase cooler efficiency. Such changes would likely be confined to minor adjustments of the phase difference between the oscillations of the volumes of the variable volume chambers.
In contradistinction thereto, the present invention provides for adjustment of the phase difference to exceed 90 degrees in order to generate heat in the cold end of the cooler. Such a large phase difference, and such use thereof, is not taught or suggested by Peterson in his disclosure.
It will thus be realized that the novel device of the present invention generates heat, when required, in the chamber, sometimes referred to as the cold end, normally used for gas expansion. This chamber has a variable volume due to displacer oscillation. Displacer oscillation has an undamped natural frequency which increases with an increase of the spring rate of the pneumatic spring and decreases with an increase in the mass of the displacer. When the piston is driven faster than this frequency, a gas pulse arrives in the expansion chamber already while the displacer is still moving towards the closed end of the chamber. Consequently, the gas, instead of being expanded as in normal operation for cooling, is now compressed by the displacer, so generating heat. Thereafter the gas is transferred back to the piston where it is expanded. Such expansion absorbs heat which is now obtained from the ambient via the same expanded surface device which in normal operation is used for rejecting heat to the ambient.
In this way the cold end of the cooler and items in thermal contact therewith can be temporarily heated to quickly remove solidified gases attached thereto. These gases are then removed by a suitable high vacuum pump. The process is analogous to removing accumulated water from a household refrigerator which has just been defrosted.
The elimination of electric heaters used on prior-art coolers for periodic fast defrosting brings advantages in reducing equipment complexity and cost, easing maintenance, and in improving safety.
The invention will now be described further with reference to the accompanying drawings, which represent by example preferred embodiments of the invention. Structural details are shown only as far as necessary for a fundamental understanding thereof. The described examples, together with the drawings, will make apparent to those skilled in the art how further forms of the invention may be realized.